In the automotive industry, aluminum alloys are used extensively for tubing because of the extrudability of the alloys as well as the combination of lightweight and high strength. Alloys for use in the automotive industry, especially for heat exchanger or air conditioning condenser applications, must have excellent strength, corrosion resistance and extrudability.
One example of a prior art aluminum alloy for use in air conditioning condensers is an AA 1000 series aluminum alloy. As a result of the improvements in automotive heat exchangers, condensers were designed with reduced wall thickness to meet the needs of new refrigerants and weight reduction. As such, the AA 1000 series materials, typically having yield stresses of about 1.5 ksi, were replaced with more highly alloyed aluminum alloys such as AA 3102, typically having a yield stress of about 2.5 ksi.
Requirements for more efficient condenser design have created a demand for aluminum alloys with strength similar to the AA 3102 type alloys but with improved corrosion resistance.
U.S. Pat. Nos. 4,649,087 and 4,828,794 describe the use of a titanium addition to an aluminum-manganese alloy to impart superior corrosion performance. The alloys described in these patents are useful for extrusions with an extrusion ratio (ratio of billet cross-sectional area to the cross-sectional area of the extrusion) less than about 200. When using extrusion ratios higher than 200, for instance a ratio on the order of 500 or more, alloys of the type described in these patents require extremely high extrusion forces to achieve these ratios. As such, these manganese, copper, and titanium containing aluminum alloys are not economical in extrusion applications with high extrusion ratios.
In view of the disadvantages of prior art alloys having superior corrosion resistance but reduced extrudability properties, and industry requirements for small cross-sectional areas and thin wall dimensions for extrusions used in condensers, a need has developed for aluminum alloy compositions having the combination of excellent extrudability and superior corrosion resistance. Excellent extrudability is required to minimize production costs at the extrusion plant, including use of lower extrusion pressures and higher extrusion speeds.
In response to this need, the present invention provides an aluminum alloy composition which exhibits superior corrosion resistance and improved extrudability. The aluminum alloy of the present invention includes controlled amounts of manganese, iron, silicon and titanium. The copper content is limited to greatly improve the extrudability of the alloy and to offset the effect of the titanium alloying component which causes the flow stress of the aluminum alloy to be higher than alloys without the addition of titanium.